With the rapid growth of our digital world, the technology for optical storage is being highly developed. The advantages of an optical medium disk are that data are easily recorded on such a medium, it is suitable for enduring storage, and it has a modest cost. This recording data medium is now becoming an accepted way to store information because of its large storage capacity.
The field of this invention is directed to dyes used in the recording layer on a writable optical disk, because such dyes exhibit varied optical properties upon irradiation. There are optical disks of several kinds as follows: (1) Write Once, Read Many (WORM); (2) Read Only Memory (ROM); (3) Erasable Direct Read after Write (EDRW). In a disk of WORM type, the recording material uses mainly dyes capable of absorbing light in the near infrared region (NIR region), as described by M. Emmelius in Angewandte Chemie, No. 11, pages 1475-1502 (1989). It is based primarily on the principle that laser irradiation of the dyes can produce the changes in absorption necessary to record information in digital form in such recording materials by means of physical (for example, by sublimation or diffusion) or chemical (for example, photochromy, isomerization or thermal decomposition of the dye) changes.
The optical recording medium disks for storage information in the market are DVD-R as 8×˜16×, and CD-R as 32×˜52×. When a CD-R operates at a large writing speed, the jitter value at short pits or lands can be decreased with a relatively thin recording layer, but this thin layer requires an increased writing power at a given writing speed, which once again limits the maximum achievable writing speed at a given laser point.
Recording materials are, however, unable to meet fully the increased requirements at great writing speeds. In particular, it is found that the optimum thickness of the recording layer varies depending on the range of writing speed. Whereas, at a small writing speed, an unsatisfactorily small contrast is generally the critical parameter that can be improved with a relative thick layer, at a large writing speed, the critical parameter is generally excessive jitter at short pits or lands (in particular L3T), which can be decreased with a relatively thin recording layer. A thin layer requires, undesirably, an increased writing power at a given writing speed, which once again limits the maximum achievable writing speed at a given laser point.
The recording layer to be used must meet demanding requirements such as a large index of refraction and small absorption at the laser wavelength, a large contrast of the written pits, a uniformity of the pit with varied pit length, great light stability in daylight and under weak laser radiation (reading) while concurrently maintaining a great sensitivity under intense laser radiation (writing), great long-term stability, small noise, high resolution and—a particularly important aspect—a minute systematic and random deviation (‘jitter’) of pit length from a prescribed value at an optimum writing power.
Among various dyes used in the optical recording medium, phthalocyanine is a widely used material and remains the most important. Phthalocyanine has the advantages of excellent light and environmental stability. The disadvantages of phthalocyanine are (1) lack of light sensitivity, (2) poor solubility, (3) a high temperature required for recording, and (4) a small reflectivity. For these reasons other substituents must be incorporated into the phthalocyanine molecular structure. U.S. Pat. Nos. 5,270,463 and 5,280,114 disclosed phthalocyanines involving bulky substituents, for example, a branched alkoxy group and halide at the same time to decrease the temperature needed for recording and to increase the solubility of the phthalocyanine dye. Although the phthalocyanines disclosed in this patent can decrease the decomposition temperature, and possess excellent writing decomposability, increased solubility in the solvent and increased reflectivity, these phthalocyanines also result in an increased Block Error Rate (BLER) when a greater writing speed is required; thus its writing property is also diminished.
EP-A 600427 disclosed a dye for a recording layer in an optical recording medium that is based on phthalocyanines with a few additives, for example, a combustion aid such as ferrocenyl derivatives, or an antiknocking agent to improve the heat decomposability and recording property, but it is still unsatisfactory for recycling because of the solubility difference between the phthalocyanines and the additives.
WO 97/23354 disclosed a method to solve the problem of the disparate solubility by binding the metallocenyl and the phthalocyanines, but its recording property is still unsatisfactory.
WO 00/09522, WO 03/068865 and WO 02/083796 disclosed binding the metallocenyl and the phthalocyanines via a bridging unit E, where E is composed of a chain of at least two atoms or atom groups selected from the group consisting of —CH2—, —C(C═O)—, —CH(C1-4 alkyl)-, —C(C1-4 alkyl)2-, —NH—, —S—, and —O—, to produce metallocenyl phthalocyanines that are capable of use in recording layers of recording media with high-speed writing.
In summary from the above-mentioned prior arts, it is generally considered that metallocenyl phthalocyanines prepared by binding phthalocyanines and metallocene together through use of an atomic group are beneficial for use in a recording layer of a high-speed rewritable recording medium and can resolve the problems encountered in the recovering/recycling process. Moreover, their solubility in solvents commonly used in this field is increased and their decomposability and rewriting stability are greatly improved.
Currently, only altering the specificity of the phthalocyanine structures themselves can not achieve a great sensitivity ratio (C/N, carrier to noise), optimal recording power and recording properties (such as small jitter) required in recording media. Also, from relevant papers and patent publications, it is pointed out that the compound obtained on bonding phthalocyanines and metallocene can greatly improve its sensitivity and recording properties when it is used as a dye in a recording layer for recording media, but the preparation of metallocenyl phthalocyanines requires multiple steps so that its cost of production becomes increased. Moreover, phthalocyanine compounds are slightly soluble in non-polar solvents commonly used in preparing recording material; thus when the phthalocyanine compounds are used in a solution to prepare the recording layer, the phthalocyanine compounds aggregate slowly and in turn precipitate after use for a period of time. A phthalocyanine compound having resolved the above problems and a simple process to prepare the same are therefore needed.